When the work roll speed is increased or the ratio of reduction is increased in a tandem cold rolling mill, heat scratches are caused on the surface of a cold-rolled sheet. A heat scratch is defined as a defect of seizing caused by metallic contact of a work roll with a sheet to be rolled when the temperature of an interface between the work roll and the sheet to be rolled is raised in a rolling bite and an oil film is broken.
When heat scratches occur on the surface of a product, the product becomes defective, and the product yield is lowered. Further, the work rolls of a cold rolling stand in which the heat scratches occur must be changed. Accordingly, the productivity of the cold rolling mill is remarkably deteriorated, which is a serious problem.
Accordingly, concerning the prevention of heat scratches, the following methods have been disclosed. For example, Japanese Unexamined Patent Publication No. 5-98283 discloses a method in which a rolling lubricant having a high anti-seizing property is used. Japanese Unexamined Patent Publication No. 56-111505 discloses a method in which a quantity of coolant is controlled so as to lower the temperatures of cold-rolled sheets and work rolls. Japanese Unexamined Patent Publication No. 6-63624 discloses a method in which the work roll speed is lowered. All of the methods relate to a technique for preventing an increase in the temperature of the interface between the work roll and the sheet to be rolled in the work roll bite and also relates to a technique for preventing a break in an oil film even when the temperature of the interface in the work roll bite is raised. However, even if the above methods are adopted the following problems may be encountered. When a rolling lubricant, the anti-seizing property of which is high, is used, there is a possibility that the production cost is raised. When the temperatures of cold-rolled sheets and work rolls are controlled by controlling a quantity of coolant, it is possible to provide an effect, however, the responding property is not so high, and further the productivity of cold rolling is deteriorated since the work roll speed is decreased.
Japanese Unexamined Patent Publication No. 60-49802 discloses a method to change a reduction schedule and tension, by which the occurrence of heat scratches can be prevented without deteriorating the productivity and increasing the production cost. However, an amount of control to be conducted by the above method is limited by the restriction placed on the apparatus of the cold rolling mill.
Fluctuation in the width of a cold-rolled sheet in the process of cold rolling is smaller than that of a hot-rolled sheet in the process of hot rolling. Therefore, conventionally, sheet width control is seldom conducted in the process of cold rolling unlike the process of hot rolling in which width gauges are arranged on the entry and the delivery side of the hot rolling mill and tension is controlled in accordance with the results of measurement of the width of a sheet measured by the width gauges.